The present invention relates to a novel process for fabricating nanopits and nanoscale patterns on surfaces of solids at room temperature in a liquid environment by applying extremely small bias voltages in an electrochemical scanning tunnelling microscopy mode. Size-controlled, location selective, and time stable nanopits may be created routinely by this method in a liquid solution without additional experimental set-up. The feasibility of formation and the stability of nanoscale pits and patterns make them of interest for potential applications in the context of information storage, as immobilisation sites for active biological molecules, and in other ways.
Nanometer and atomic-scale modifications of materials have long been desired from both scientific and technical points of view. However, direct visualisation on the nanometer scale could not come true until the first half of this century with the invention of the electron microscope. The advent of the local probe microscopes, particularly the scanning tunneling microscope (STM) has advanced imaging and measuring to the atomic level since the 1980s. Furthermore, STM can serve as a useful tool, not only imaging with unprecedented resolution but controlling and fabricating nanostructures in the nanoworld.
One of the most striking examples is to create new functional structures with nanometer or atomic scale on solid surfaces by the STM through its three main operations: manipulation, removal, and deposition.
Becker and co-workers (Becker et al., Nature (London) 325 (1987) 419) first reported an atomic-scale modification of Ge(111) with the STM. Following this initial achievement, Eiger et al. showed many more exciting results by using STM. For instance, they deposited individual Xe atoms on the Ni(110) surface to create various patterns and constructed quantum corrals from single atoms (Eiger et al., Nature (London) 344 (1990) 524; Science 254 (1991) 1319; Science 262 (1993) 218). These results show that single atoms can be manipulated and desired structures at an atomic level built. These pioneering studies were done in ultra high vacuum (UHV) at low temperature (4 K) where atoms were xe2x80x98frozen-inxe2x80x99 so as to keep them from moving around owing to their thermal energy.
The nanoscale modification of solid surfaces at ambient temperature condition either in UHV or in air was subsequently explored. Successful examples include nanofabrication of mounds and/or pits on different materials such as Si, HOPG, metals, semiconductors, and superconductors (Lyo et al., Science 253 (1991) 173. Kobayashi et al., Science 259 (1993) 1724. Albrecht et al., Appl. Phys. Lett. 55 (1989) 1727. Lebreton et al., Microelectron. Eng. 30 (1996) 391. Sugimura et al., J. Phys. Chem. 98 (1994) 4352. Huang et al., Appl. Phys. Lett. 61 (1992) 1528. Hosaka et al., J. Vac. Sci. Technol. B13 (1995) 2813. Thompson et al., Nanotechnology 5 (1994) 57). In addition, it has been demonstrated that individual molecules can be manipulated with STM tips at room temperature without disruption of the molecular structure.
Common to these investigations is that the appropriate nanostructures are implemented by large (i.e. several volts) bias voltages, either in continuous or in pulse modes. Extension of the working environment to metal/liquid solution interfaces would strongly broaden the perspectives for nanofabrication of surface structures. A wealth of solute molecules and ions could thus be starting materials in controlled adsorption and electrochemical electron transfer modes. In comparison with nanoscale modification in UHV reports of controlled nanofabrication in aqueous electrochemical environments are, however, few in numbers.
Extension of STM to aqueous and other conducting liquids requires operation in the electrochemical, or in situ mode. Control of the separate potentials of the substrate and STM tip, relative to a common reference electrode is here essential. Otherwise the tunnel current is entirely hidden by much larger Faradaic currents associated with uncontrolled solvent decomposition, metal dissolution and deposition processes etc. In addition insulating material except for the outermost end must cover the tip. This is because the Faradaic currents follow the exposed electrode area while the tunnelling current is independent of the area and carried only by a small tip region closest to the substrate surface. Penner et al. (Penner et al., Appl. Phys. Lett. 60 (1992) 1181) first attempted to use electrochemical STM for the deposition of nanoscale metal clusters on HOPG.
The fabrication of nanopits on Si (100) under electrochemical environment was also attempted by Ye et al. (J. Vac. Sci. Technol. B13 (1995) 1423), but the control in dimensions and locations of pits was not satisfactory and demonstrated to be more difficult than in air or in UHV. Kolb, and associates (Kolb et al., Chem. Phys. Lett. 209 (1993) 239; Ber. Bunsenges. Phys. Chem. 99 (1995) 1414; Science 275 (1997) 1097. Engelmann, J. Electrochem. 145 (1998) L33) have more recently developed a procedure for depositing locally nanoscale clusters of Cu and Pd on Au(111) surfaces using in situ STM and well defined electrochemical conditions. The Au(111) and tip potentials were initially chosen in such a way that the Au(111) surface was covered by a layer of underpotential deposited copper, and excess copper was also present on the tip. A 50-90 mV potential step temporarily inverts the potential bias. This takes the tip close enough to the surface that a cluster of copper or palladium atoms is transferred from the tip to the surface.
In U.S. Pat No. 4,896,044 a method resting on a two-electrode configuration (substrate and tip) operated in air was presented. Claimed extension to operation in liquids does not apply to conducting liquids such as water where, as noted double potentiostatic control and tip coating is essential. Topographic depressions in the Au(111) surface are implemented when the bias potential in the constant current mode is stepped from 0.65 V to a value of at least 2.7 V in 500 xcexcs pulses. This raises the tunnel current from 1 nA to values between 10 and 100 nA which is sufficient to evaporate material from the substrate. There is no evidence of material about the orifice of the craters but hillocks with no craters are formed on longer time exposure to large tunnel currents or bias voltages.
It should be noted that the claimed extension of the method in U.S. Pat. No. 4,896,044 to liquids is not documented and that the nature of the liquid is not specified. Furthermore, the claimed mechanism involving metal evaporation in U.S. Pat. No. 4,896,044 is reputed to be still controversial.
It is a disadvantage of the method in U.S. Pat. No. 4,896,044 that it only works in a nonconducting environment.
It is a further disadvantage of the method in U.S. Pat. No. 4,896,044 that a relative high bias voltage (at least 2,7 V) has to be applied in order to evaporate material from the substrate making the method in U.S. Pat. No. 4,896,044 less controllable and therefore less attractive seen from an industrial point of view.
It is an object of the present invention to provide a method for fabricating nanopits and nanoscale patterns in a conducting liquid environment.
It is a further object of the present invention to provide a method wherein the fabricating of nanopits and nanoscale patterns can be well controlled regarding pits-size and nanoscale pattern reproducibility.
The above-mentioned objects are complied with by providing a method for forming at least one nanoscale depression in a surface of a substrate, said method comprising the steps of:
immersing at least part of the surface of the substrate into a liquid environment,
immersing at least part of an object into the liquid environment,
bringing the object within proximity of the surface of the substrate by applying a first set of operation parameters, said first set of operation parameters comprising bias voltage, tunnel current and working potential, and
forming at least one nanoscale depression in the surface of the substrate by applying a second set of operation parameters, said second set of operation parameters comprising bias voltage, tunnel current and working potential, wherein the bias voltage of the second set of operation parameters is negative.
The bias voltage is applied between the object and the surface of the substrate. As mentioned above, the bias voltage of the second set of operation parameters is negative. By negative is meant that, the potential of the object is lower than the potential of the surface of the substrate. The object may form part of a tip of a scanning probe microscope, such as a scanning tunneling microscope.
The bias voltage of the second set of operation parameters may be in the range xe2x88x9210 mV-0 V, preferably in the range xe2x88x928 mV-0 V, more preferably in the range xe2x88x924 mV-0 V, such as approximately xe2x88x922 mV. The tunnel current of the second set of operation parameters may be in the range 0-10 nA, preferably in the range 0-6 nA, more preferably in the range 0-4 nA, even more preferably in the range 2-3 nA.
The working potential of the second set of operation parameters may be in the range xe2x88x920.1-0.5 V vs NHE, preferably in the range 0-0.4 V vs NHE, more preferably in the range 0.1-0.3 V vs NHE.
The bias voltage of the first set of operation parameters may be in the range 100-300 mV, and wherein the tunnel current of the first set of operation parameters is in the range 0.5-5 nA, and wherein the working potential of the first set of operation parameters is in the range 0.1-0.6 V vs NHE.
The liquid environment may be an electrically conducting environment, which may comprise an aqueous acidic, basic, neutral, or salt containing electrolyte solution such as HClO4. The concentration of the HClO4 solution may be in the range 10xe2x88x923-10 M, preferably in the range 0.02-0.4 M, more preferably in the range 0.03-0.3 M and even more preferably in the range 0.05-0.1 M. Alternatively or in addition, the liquid environment may comprise chloride, sulphate, or other adsorbing inorganic or organic anions or molecules.
Part of the surface of the substrate may hold an electrically conducting material, such as a metal, so that the at least one depression is formed in said electrically conducting material. Part of the surface of the substrate may hold a gold film, or constitutes bulk gold. Part of the surface of the substrate may hold a semiconductor material, such as silicon.